This invention relates to the separation of coal from its associated mineral matter, resulting in nearly pure coal and less pollution potential.
Most coal naturally contains some inorganic mineral matter in the form of small particles which are widely disseminated throughout the coal structure. The mineral matter generally includes various types of clay, silica, carbonate minerals, and iron pyrite. It may also contain toxic trace elements such as mercury. When coal is burned, the mineral matter is largely converted to metal oxides in the form of ash. However, the sulfur is released as sulfur oxides, and mercury is also volatilized. While it is advantageous to burn clean coal in order to limit environmental pollution, highly cleaned coal is seldom available because of the limitations of present coal cleaning methods.
Physical coal cleaning requires crushing the material to liberate the mineral particles, followed by particle separation. Coarse particles are readily separated by methods which take advantage of the difference in density of the organic material and the inorganic minerals. Fine particles are much more difficult to separate, and are generally separated by methods based on surface properties. The most commonly employed fine particle separation method is froth flotation. In this method, fine hydrophobic coal particles in an aqueous suspension become attached to gas bubbles which rise to the surface of the suspension and are collected in a thick layer of froth which is skimmed off. Most mineral particles are hydrophilic and remain in the aqueous suspension. The optimum particle size for froth flotation appears to be between 50 and 140 mesh (0.3 mm and 0.105 mm). However, newer versions of the method employ tall flotation columns and can treat coal particles having a mean diameter of about 25 xcexcm.
A promising alternative fine particle separation process is one based on selective oil agglomeration of coal particles in an aqueous suspension. Almost any hydrocarbon liquid which is completely immiscible with water can be used to agglomerate the coal. If a large amount of oil is used (e.g., 30 to 50% based on coal weight), relatively large agglomerates are produced which can be recovered on a screen. The method can be used to recover particles which are much smaller than those recoverable by froth flotation. By grinding coal to micrometer size and selectively agglomerating the organic particles with a large amount of pentane, super clean coal has been produced experimentally. Although oil agglomeration methods are technically feasible, they have seldom been used commercially because of the cost of oil.
In summary, disadvantages with froth flotation are that the particle sizes are generally required to be larger than occurs with some coal fines, and disadvantages of the oil agglomeration process include that it requires significant amounts of costly oils. There is a need, therefore, for a process which can be used with very fine particles to separate mineral matter from coal, and for a process which does not involve use of large amounts of agglomerating oil.
Several years ago in our research we demonstrated an alternative agglomeration method in which hydrophobic particles in an aqueous suspension are bound together by small gas bubbles to form agglomerates (J. Drzymala and T. D. Wheelock, xe2x80x9cAir agglomeration of hydrophobic particles,xe2x80x9d in: Processing of Hydrophobic Minerals and Fine Coal, J. S. Laskowski and G. W. Poling (eds.), Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, 1995, pp. 201-211). We found that various hydrophobic materials, including Teflon, gilsonite, graphite and sulfur can be agglomerated by this method. Further, coal which had been treated with a small amount of heptane to make its surface more hydrophobic could also be agglomerated. We then found a brief mention of a similar form of agglomeration by A. F. Taggert, (Elements of Ore Dressing, Wiley, New York, 1951). However, in spite of the fact that the phenomenon of agglomeration of oiled mineral particles by small gas bubbles was reported long ago, it does not appear to have been developed or used in a reversible multi-stage process.
From the above description it can be seen that there is a real and a continuing need for a process which overcomes the disadvantages of froth flotation separation of minerals from coal fines, and the disadvantages of oil agglomeration processes. In particular, there is a real and a continuing need for a process which can effectively separate minerals from very fine coal particles without the need for use of large amounts of agglomerating oil. This invention has as its primary objective the fulfillment of this need.
Another objective of the present invention is to provide a gaseous agglomeration of coal particles in an aqueous suspension by a process which allows extremely small particles to be separated without requiring much agglomerating oil.
A further objective of the present invention is to provide a process meeting the above-described objectives which can be practiced on either a batch or a continuous multi-stage process.
The method and manner of accomplishing each of the above objectives as well as others will become apparent from the detailed description of the invention which follows hereinafter.